1. Field of the Invention
The present invention relates generally to the field of airfoil repair. More particularly, the present invention relates to systems and methods for measuring airfoil shape to be repaired, comparing the measured airfoil to a nominal shape, and generating a numerically-controlled (NC) tool path to blend a weld-repaired region smoothly with its adjacent surfaces.
2. Description of the Related Art
Airfoils of gas turbine engines are regularly exposed to harsh operating conditions, such as foreign object damage (FOD), high operating temperatures, vibrations and significant pressure fields, as part of normal operating conditions of the engine. For metallic airfoils disposed within an engine, the extreme thermal cycling of normal engine operation represents one of the more deleterious conditions that amplifies the already severe circumstances under which the airfoils must operate. Typically, these operating conditions act over time to deteriorate and weaken the airfoils, making them susceptible to the formation of damage areas such as cracks, pits and depressions. Airfoils must be periodically inspected, repaired and even replaced. If damage is not properly addressed and repaired correctly, the airfoils may become irreparably damaged as a result of the rapid propagation of existing damage. In addition, if not properly repaired, cracks and other structural weaknesses may lead to other engine component failure.
It is known that turbine blades may develop one or more cracks near the tip of a blade due to low cycle fatigue stresses imparted on the blade tip during the operation of the turbine. If a crack extends beyond a critical dimension, the turbine blade must be removed from service and/or repaired in order to prevent catastrophic failure of the blade and turbine. It can be appreciated that a crack may be repaired by removing the material adjacent to the crack to form a crack repair volume, and then filling the crack repair volume with weld metal. Welding allows the service life of the blade to be extended, saving the time and costs associated with replacing the part.
Various conventional methods for repairing airfoils involve the use of a coordinate measurement machine (CMM) or a specialized laser scanner to measure the blade. A CMM uses a mechanical probe that traverses a path covering the surface of the article to be measured. CMM machinery typically works with algorithms that recognize part features, such as edges, from user supplied nominal locations. A CMM process, however, is slow, interactive and operator intensive. Laser scanning is technique used to collect data points from the surface of a 3-dimensional article. The data points correspond to coordinate values over the surface of the scanned article, e.g. taken along the x, y, and z axes. A laser scan traverses a scan path over the surface of an article, and at selectable distances along the path point values may be taken. A laser scan generally results in the generation of a set of scan data in a digitized format. Laser scanning is limited by the great magnitude of data that must be processed relative to other measuring techniques. Another factor is the limited ability of laser scan software algorithms to recognize or differentiate surface transition features, such as corners, vertices, openings and boundaries. Still another factor is the difficulty in correlating the scan path of the laser head to a desired tool path for a numerically-controlled (NC) machine tool. A correlation between the laser scan path and the NC tool cutting path may result in gouging of the article. These limitations become pronounced when laser scanning is used for capturing the geometry of complex parts such as a blade. An airfoil of a blade is typically a complex free-form shape and requires many measurements to accurately define its shape.
In addition, while these repair techniques have been developed to repair damage, these methods are not suitable for on-machine measurement because they demand additional set-ups. They also require reverse engineering software to reconstruct the blade geometry using mathematical models such as spline surfaces. Reconstructing the blade geometry using spline surfaces tends to create mathematical artifacts. These artifacts can introduce dimensional errors in the final numerically-controlled (NC) tool path that is generated from the spline surfaces.
It is costly to replace used turbine blades with brand new blades, since these precision parts are made of expensive, high-temperature materials requiring complicated processing and machining operations. A lower-cost alternative is to repair only the region of the blade that has worn out or been damaged. It is therefore imperative that repair strategies be developed to facilitate a renewal of the engine component that restores it to a physical condition resembling its original state. What are needed are integrated systems and methods that can measure and machine airfoil blades that are weld-repaired.